1. Field of the Invention
This invention relates to an image recording apparatus for recording an image by irradiating a recording material with light beams emitted from light sources.
2. Description of the Related Art
Such an image recording apparatus includes a recording drum rotatable about its axis with a recording material mounted peripherally thereof, and a recording head movable parallel to the axis of the recording drum and emitting light beams, thereby scanning the recording material with the light beams.
Such an image recording apparatus uses light sources such as semiconductor lasers or LEDs. To meet the requirement for an increased speed of image recording today, the number of semiconductor lasers or LEDs used in one image recording apparatus is on the increase, and thus a light source unit has been enlarged. In this connection, cooling of the light source unit has become important, and this further enlarges the light source unit.
For this reason, image recording apparatus have been proposed in Japanese Unexamined Patent Publications No. 2000-141724 and No. 2003-237128, in which light emitted from light sources fixed to a main body of the apparatus is guided by optical fibers to a recording head moving relative to a recording material.
In such image recording apparatus, one end of each optical fiber is fixed to the recording head. The other end of each optical fiber is connected to the light source fixed to the main body of the apparatus. Therefore, with movement of the recording head, the optical fibers also move while undergoing deformation. However, when the optical fibers deform in time of recording an image, speckles of the light beams move at the exit end with variations in the mode of light propagation. This could cause a problem of lowering the quality of the image formed.
That is, optical fibers can be classified into single mode optical fibers that transmit light in only one mode, and multi-mode optical fibers having a mixture of different transmitting modes. An image recording apparatus, since the light sources such as high-output semiconductor lasers or LEDs have large emitters, uses multi-mode optical fibers having a core diameter of at least about 50 μm, rather than single mode optical fibers having a core diameter of 5 μm or less.
The multi-mode optical fibers can be classified into step index type multi-mode optical fibers with a core having a constant refractive index, and graded index type multi-mode optical fibers with a core having a smooth distribution of refractive indexes.
An image recording apparatus in which light emitted from light sources fixed to a main body of the apparatus is guided by optical fibers to a recording head moving relative to a recording material may use step index type multi-mode optical fibers. In this case, when the optical fibers deform in time of recording an image, as noted above, speckles of the light beams will move at the exit end with variations in the mode of light propagation. This could cause a problem of lowering the quality of the image formed.
On the other hand, graded index type multi-mode optical fibers used in this type of image recording apparatus can eliminate the above problem of speckles of the light beams moving at the exit end with variations in the mode of light propagation. However, light will gather in central parts of the graded index type multi-mode optical fibers, and the degree of gathering is variable depending, for example, on affinity with the light sources. The degree of light gathering may not fully be controlled, leading to a problem of variations in image quality.
Japanese Unexamined Patent Publication H8-145630 (1996), though in a different technical field to this invention, discloses a technique of using graded index type multi-mode optical fibers and step index type multi-mode optical fibers. That is, this publication discloses an apparatus for monitoring a building or land, including support plates fixed to both sides of a part to be measured of the building or land, multi-mode optical fibers supported by the support plates, and light sources and light receivers with light attenuation evaluating units connected to opposite ends of the multi-mode optical fibers. The multi-mode optical fibers include a plurality of graded index type multi-mode optical fibers GI forming arcuate curved parts, and step index type multi-mode optical fibers SI connecting the step index type multi-mode optical fibers GI and having loop parts.